Infections that are present at the same time as HIV infection. They may be acquired before, after or at the same time as HIV. Co-infections may contribute to the progression of HIV infection or can be risk factors for HIV transmission. Some co-infections complicate the treatment of HIV and may become more difficult to treat over time and symptoms may increase in severity. Hepatitis B and C are common HIV co-infections. Other sexually transmissible infections (STIs) can also occur as co-infections. Many co-infections can be effectively treated and cured, but some cannot (for example herpes or HPV infection). Many co-infections can be prevented by reducing risks associated with acquiring them (such as hepatitis C) and some are vaccine preventable (such as hepatitis A and B).